1. Field of the Invention
The present invention relates to monomer compositions containing
A) vinyl-containing monomers in which a heteroatom selected from the group consisting of the halogens, nitrogen, oxygen, sulfur and silicon is present on the vinyl group, and
B) at least one N-oxyl compound of a secondary amine which carries no hydrogen atoms on the xcex1-carbon atoms, in an amount effective as stabilizer for preventing premature polymerization.
2. Description of the Background
The present invention furthermore relates to processes for inhibiting the premature polymerization of vinyl-containing monomers and to the use of N-oxyl compounds of secondary amines which carry no hydrogen atoms on the xcex1-carbon atoms as stabilizers for preventing premature polyemrization.
It is necessary to add stabilizers to the monomers in order to prevent premature polymerization. Sterically hindered amines, such as 2,2,6,6-tetraalkylpiperidine and derivatives thereof, including the N-oxyl derivatives, have proven particularly suitable for preventing free radical polymerizations.
U.S. Pat. No. 5,254,760 describes the stabilization of vinylaromatic compounds, such as styrene, during distillation and purification by a combination of at least one nitroxyl compound and at least one aromatic nitro compound. There is a danger here that traces of nitroxyl compounds will enter the purified monomers. However, even traces of nitroxyl compounds interfere with the subsequent polymerization; they cause delayed polymerization and uncontrolled chain terminations, leading to polymers having poor reproducibility and short chain lengths. These disadvantageous effects are described by Mardare et al. in Polym. Prep. (Am. Chem. Soc., Div. Polym. Sci.) 35 (1), (1994) 778.
For example, derivatives of phenylenediamine (U.S. Pat. No. 5,396,005), fullerenes (DE-A 44 14 773) or 2,6-di-tert-butyl-p-cresol (DE-A 43 28 950) have been used to date for stabilizing hetero-substituted vinyl compounds, such as N-vinylformamide or N-vinylpyrrolidone, during distillation and purification. However, these stabilizers are unsatisfactory with regard to their effectiveness.
It is an object of the present invention to provide monomer compositions which contain hetero-substituted vinyl compounds and suitable stabilizers, exhibit improved stabilization to premature polymerization and scarcely have any adverse effect on the subsequent specific polymerization of the monomers.
We have found that this object is achieved by the monomer compositions stated at the outset.
The hetero-substituted vinyl monomers preferably carry a halogen, oxygen, nitrogen or sulfur as the heteroatom on the vinyl group.
Examples of suitable hetero-substituted vinylmonomers are vinyl halides, such as vinyl chloride, vinyl carboxylates, such as vinyl acetate, vinyl propionate or vinyl butyrate, vinyl ethers, such as methyl vinyl ether, ethyl vinyl ether or butyl vinyl ether, vinyl thioethers, vinylcarbazoles, vinylpyrrolidones, vinylphthalimides, vinyl isocyanates, vinylcaprolactams, vinylimidozoles, vinylformamide, vinylsulfonic acid and vinylsilanes, such as vinyltriacetoxysilane, vinyltrichlorosilane or vinyltrimethoxysilane.
Preferred monomer compositions contain
A) monomers of the general formula I
CH2xe2x95x90CHxe2x80x94Xxe2x80x94R1xe2x80x83xe2x80x83I,
xe2x80x83where
X is oxygen or xe2x80x94NR2xe2x80x94,
R1 is 
xe2x80x83or xe2x80x94R3,
R2 is hydrogen or C1-C4-alkyl or, together with R3, is a saturated or unsaturated C3-, C4- or C5-alkylene bridge in which one or two CH2 groups may be replaced by NH, N(C1-C4-alkyl), N(C6-C10-aryl) or oxygen and one or two CH groups may be replaced by N, and
R3 is hydrogen, C1-C4-alkyl or a radical which, together with R2, is a saturated or unsaturated C3-, C4- or C5-alkylene bridge in which one or two CH2 groups may be replaced by NH, N(C1-C4-alkyl), N(C6-C10-aryl) or oxygen and one or two CH groups may be replaced by N, and
B) at least one N-oxyl compound of a secondary amine which carries no hydrogen atoms on the xcex1-carbon atoms, in an amount effective as a stabilizer for preventing premature polymerization.
In the monomers (A) of the general formula I which are contained in the novel mixtures, X may be oxygen. Among these monomers, the vinyl ethers in which R1 is C1-C4-alkyl, i.e. methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl or tert-butyl, are particularly suitable as a component of the novel monomer compositions.
If X is xe2x80x94NR2xe2x80x94, R1 is preferably xe2x80x94COxe2x80x94R3.
Other suitable radicals R3 in addition to hydrogen and the stated C1-C4-alkyl groups are those radicals which, together with xe2x80x94NR2xe2x80x94, form a saturated or unsaturated 5- to 7-membered ring. Examples of such ring systems are: 
in particular the N-pyrrolidinonyl and the N-caprolactamyl radical.
Preferred monomers in the novel compositions are N-vinylformamide, N-vinyl-2-pyrrolidone, N-vinyl-xcex5-caprolactam and the abovementioned C1-C4-alkyl vinyl ethers.
Particularly preferred among these monomers is N-vinylformamide.
The novel monomer compositions contain, as stabilizers (B), at least one N-oxyl compound of a secondary amine which carries no hydrogen atoms on the xcex1-carbon atoms. These compounds may be present as free compounds or in the form of their salts.
Suitable N-oxyls of amines are, for example, the following structures 
where R are identical or different alkyl, cycloalkyl, aralkyl or aryl radicals which may also be bonded in pairs to form a ring system, and Y is a group which is required to complete a 5- or 6-membered ring. For example, R is C1-C20-alkyl, in particular C1-C8-alkyl, C5- or C6-cycloalkyl, benzyl or phenyl. Y is, for example, alkylene -(CH2)2xe2x80x94 or xe2x80x94(CH2)3xe2x80x94.
N-Oxyl compounds such as the following structures 
where the aromatic rings may each furthermore carry from 1 to 3 inert substituents, for example C1-C4-alkyl, C1-C4-alkoxy or cyano, are also suitable.
Sterically hindered derivatives of cyclic amines, for example of piperidine or pyrrolidine compounds, which may contain a further heteroatom, such as nitrogen, oxygen or sulfur, in the ring, are preferably used, this heteroatom not being adjacent to the hindered amine nitrogen. The steric hindrance is achieved by substituents in two positions adjacent to the amine nitrogen, suitable substituents being hydrocarbon radicals which replace all four hydrogen atoms of the xcex1-CH2 groups. Examples of such substituents are phenyl, C3-C6-cycloalkyl, benzyl and in particular C1-C6-alkyl, where the alkyl radicals bonded to the same xcex1-carbon atom may furthermore be bonded to one another to form a 5- or 6-membered ring. The radicals mentioned specifically under R4 and R5 are particularly preferred. Preferably used N-oxyls of sterically hindered amines are derivatives of 2,2,6,6-tetraalkylpiperidine.
Preferred N-oxyl compounds in the novel monomer compositions are those of the general formula II 
where
R4 and R5 are each C1-C4-alkyl or phenyl or, together with the carbon atom to which they are bonded, are a 5- or 6-membered saturated hydrocarbon ring,
R6 is hydrogen, hydroxyl, amino or an m-valent organic radical bonded via oxygen or nitrogen or, together with R7 is oxygen or a ring structure defined under R7,
R7 is hydrogen or C1-C12-alkyl or, together with R6 is oxygen or, together with R6 and the carbon atom to which they are bonded, is one of the following ring structures 
xe2x80x83where, when R6 forms a common radical with R7; m is 1,
R8 is hydrogen, C1-C12-alkyl or xe2x80x94(CH2)zxe2x80x94COOR9,
R9 are identical or different C1-C18-alkyl radicals,
k is 0 or 1,
z and p are each from 1 to 12 and
m is from 1 to 100.
R4 and R5 may be, for example, the C1-C4-alkyl groups stated for R1 or together they may form a tetra- or pentamethylene group. R4 and R5 are each preferably methyl.
Examples of suitable radicals R7 are hydrogen, the abovementioned C1-C4-alkyl groups and pentyl, sec-pentyl, tert-pentyl, neopentyl, hexyl, 2-methylpentyl, heptyl, 2-methylhexyl, octyl, isooctyl, 2-ethylhexyl, nonyl, 2-methylnonyl, isononyl, 2-methyloctyl, decyl, isodecyl, 2-methylnonyl, undecyl, isoundecyl, dodecyl and isododecyl, (the names isooctyl, isononyl and isodecyl are trivial names and originate from the carbonyl compounds obtained by the oxo synthesis; in this context, cf. Ullmann""s Encylopedia of Industrial Chemistry, 5th Edition, Vol. A1, pages 290-293, and Vol. A10, pages 284 and 285).
p is preferably 6-12, particularly preferably 9.
z is preferably 1-4, particularly preferably 2.
Suitable radicals R8 in addition to hydrogen are, for example, the abovementioned C1-C12-alkyl groups. R8 is preferably hydrogen, C1-C4-alkyl or (CH2)zxe2x80x94COO(C1-C6-alkyl), particularly preferably xe2x80x94CH2xe2x80x94CH2xe2x80x94COO(CH2)11xe2x80x94CH3 and xe2x80x94CH2xe2x80x94CH2xe2x80x94COO(CH2)13xe2x80x94CH3.
R9 may be, for example, one of the abovementioned C1-C12-alkyl groups or tridecyl, isotridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl or octadecyl. Dodecyl and hexadecyl are preferred.
Preferred radicals R6 are, for example, the following m-valent radicals 
where
R10 is C1-C12-alkyl or xe2x80x94(CH2)zxe2x80x94COOR9,
R11 is hydrogen or C1-C18-alkyl,
R12 is C1-C18-alkyl, vinyl or isopropenyl,
R13 is C8-C22-alkyl,
R14 is hydrogen or an organic radical as usually formed in the free radical polymerization of the starting monomers,
k is 0 or 1,
x is from 1 to 12 and
n is an even number m.
If R6 is one of these radicals, R7 is preferably hydrogen. m may then be from 1 to 100. m is preferably 1, 2, 3, 4 or from 10 to 50, mixtures generally being used particularly in the case of the oligomeric or polymeric radicals R6.
Suitable radicals R10 are those radicals stated for R8. R10 is preferably C1-C4 -alkyl.
Suitable radicals R11 in addition to hydrogen are those radicals which have been stated for R9. R11 is preferably hydrogen.
Particularly suitable radicals R12 are vinyl, isopropenyl and C15-C17-alkyl.
Examples of suitable radicals R13 are the abovementioned C8-C18-alkyl radicals and nonadecyl, eicosyl, uneicosyl and doeiosyl. Mixtures of different radicals R13 which differ in the length of the carbon chain are preferred.
R14 is hydrogen or an organic radical as formed in the free radial polymerization of the starting monomers, in this case of an ethylene derivative and a maleimide derivative, i.e. a radical which is formed from the polymerization initiator or from an intermediate radical, or another radical of this type which is familiar to the person skilled in the art.
Other preferred stabilizers in the novel monomer compositions are compounds or mixtures of compounds of the general formula IIa 
where R15 is methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl, sec-butyl, tert-butyl, fluorine, chlorine or trimethylsilyl. The stabilizers of the formula IIa are particularly suitable for monomer compositions which are to be purified by distillation.
Preferred nitroxyl compounds as component B) of the novel monomer compositions are also the following:
1-oxyl-2,2,6,6-tetramethylpiperidine,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-ol,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-one,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl acetate,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 2-ethylhexanoate,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate,
1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 4-tert-butylbenzoate,
bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)succinate,
bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipate,
bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)sebacate,
bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)n-butylmalonate,
bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)phthalate,
bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)isophthalate,
bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)terephthalate,
bis (1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)hexyhydroterephthalate,
N,Nxe2x80x2-bis-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,Nxe2x80x2-bisformyl,-1,6-diaminohexane
N,Nxe2x80x2-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipinamide,
N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)caprolactam,
N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)dodecylsuccinimide,
2,4,6-tris-[N-butyl-N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl]-s-triazine,
4,4xe2x80x2-ethylenebis(1-oxyl-2,2,6,6-tetramethylpiperazin-3-one) and
tris(2,2,6,6-tetramethyl-1-oxyl-piperidin-4-yl)phosphite.
The nitroxyl compounds described can be prepared from the corresponding piperidine compounds by oxidation, for example with hydrogen peroxide. Details of this oxidation are given, for example, in the prior German Patent Application 195 101 84.7. The secondary amine which carry no hydrogen atoms on the xcex1-carbon atoms, such as piperidine compounds, and their preparation are generally known. Since the oxidation reactions do not always go to completion, the piperidine compounds used as starting compounds as well as partially oxidized intermediates may be present in the novel monomer compositions.
Particularly suitable monomer compositions are those which contain one or more aromatic nitroso or nitro compounds in addition to the stated nitroxyl compounds for stabilization.
For example, 1,3-dinitrobenzene, 1,4-dinitrobenzene, 2,6-dinitro-4-methylphenol, 2-nitro-4-methylphenol, 2,4,6-trinitrophenol, 2,4-dinitro-1-naphthol, 2,4-dinitro-6-methylphenol, 2,4-dinitrochlorobenzene, 2,4-dinitrophenol, 2,4-dinitro-6-sec-butylphenol, 4-cyano-2-nitrophenol, 3-iodo-4-cyano-5-nitrophenol, particularly preferably 2,6-dinitro-4-methylphenol, 2-nitro-4-methylphenol, 2,4-dinitro-6-sec-butylphenol and 2,4-dinitro-6-methylphenol may be used as aromatic nitro compounds.
Examples of suitable aromatic nitroso compounds are p-nitrosophenol, p-nitroso-o-kresol and p-nitroso-N,Nxe2x80x2-diethylaniline.
Furthermore, substituted phenols may also be added to the monomer compositions as costabilizers, for example the following substituted phenols:
4-tert-butylpyrocatechol, methoxyhydroquinone, 2,6-di-tert-butyl-4-methylphenol, n-octadecyl xcex2-(3,5-di-tert-butyl-4-hydroxy-phenol)propionate, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenol)butane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl) isocyanurate, 1,3,5-tris[xcex2-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxyethyl]isocyanurate, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate and pentaerythrityl tetrakis[xcex2-(3,5-di-tert-butyl-4-hydroxy-phenyl)propionate].
The novel monomer compositions may furthermore contain one or more costabilizers selected from the group consisting of the phenothiazines, quinones, hydroxylamines and phenyldiamines.
For the stabilization of the novel monomer compositions, the latter contain in general from 0.0002 to 5, preferably from 0.0005 to 0.5, % by weight, based on the total amount of the monomer composition, of the nitroxyl compounds.
The stabilizers display their stabilizing effect in a broad temperature range. They are effective at any conventional storage temperature from xe2x88x9250 to +50xc2x0 C. and also at elevated temperatures, as used, for example, in the distillation of the monomers. The pressure range of the stabilization process is also not critical. The stabilizers are effective at atmospheric pressure and also at reduced pressure, as used in some distillation processes.
The novel process for inhibiting the premature polymerization of monomers is used during the preparation, the distillation or the purification of the monomers and also during their storage and transport. Particularly in the distillation, the use of small N-oxyl compounds which, during the distillation, have a sufficient vapor pressure to display their action in the vapor space as well is particularly advantageous. The N-oxyl compounds of the formula IIa are particularly suitable for this purpose.